Electrical contact



June 30, 1931. E Z N 1,812,495

ELECTRICAL CONTACT Filed Jan. 51, 1929 Patented June 30, 1931 I UNITEDSTATES PATENT OFFICE HECTOR BL'BWA, OI FLINT, MICHIGAN, ASSIGNOR TO A. CSPARK PLUG COMPANY, OI FLINT, MICHIGAN, A. COMPANY 01' MICHIGANELECTRICAL CONTACT Application filed January 81, 1929. Serial Io.336,470.

This invention relates to contacts such as are employed for switches,make and break devices, relays and the like. Such contacts are subjectto rapid deterioration in service 5 as a result of the sparking whichtakes place particularly when the contact polnts separate. This sparkingcontinues until the resistance across the air gap which separates thepoints is higher than the applied voltage. It is well known that tominimize the mjurious effects of the spark, it is desirable to shortenits duration as much as possible.

For this reason snap-switches are used that open and close the circuitat high speed. Where heavy currents are encountered, magnets are used inan effort to quickly quench the arc. In other cases condensers areemployed to reduce sparking.

My invention has to do with greatly reducing the duration of the sparkand the 1njurious effects of sparking by properly selecting andfabricating the matenals of which the contacts are made.

It has heretofore been generally accepted that the sparkin voltageacross the separating contacts am? hence the duration of the spark isindependent of the material of which the contacts are composed. On thecontrary, I have found that the duratlon of the spark is greatlylengthened by the presence in the contact metal of certain impurities.which have heretofore escaped observation, and which markedly affect thesparking voltage. I have found this to be the case even where theimpurities are present in such small amounts as to be incapable ofdetection by chemical analysis.

The impurities to which I refer comprise chemical elements which arecharacterized by the ease with which they give off electrons. This isoften otherwise expressed b stating that the elements have a low workunction; that is, that but a relatively small amount of work is requiredto remove an electron from the element. Work function may be measured bymeasuring the thermo-ionic emission and in various other ways. The classof elements having a low work function are in general electro-positiveand include the alkali and alkaline earth metals. Specifically the groupcomprises lithium, sodium, potassium, rhubidium, caesium, beryllium, maesium calcium, strontium, barium and Pa ium. Ii have found that thepresence in the contacts of any of these elements, whether as elements.alloys, or comdpounds, even in slight traces, results in a re notion ofthe sparking voltage and consequently in increased duration of the sparkand greater destructive effect. Not only is this the case, but thesematerials likewise produce a great variation in the sparkinivoltage fromspark to spark since the distr1 ution of the impurities throughout thecontact is often not uniform, and since the percentage of the impuritiesdecreases with the increased use of the contact for the reason that thelow work function elements leave the contacts at a faster rate than thehigh work function elements.

According to my invention I so select the contact materials, and sotreat them in manufacture as to insure that none of the elements of lowwork function shall be present even in the smallest amounts. Theirpresence may be detected by spectroscopic analysis, and by studies ofthermo-ionic emission, and in case such analysis should reveal them tobe present in objectionable amounts, I may effect their removal by asuitable process such as by cooking the contact metal under high vacuumas explained in the following specification.

With substantially all traces of the elements of low work functionremoved from the contact material, the duration of the sparking will begreatly lessened, and the contacts will have greatly lengthened lifewith more uniform sparking voltage. This will make it possible tosubstitute less expensive1 metals or alloys for the materials now use Onthe drawings I have shown diagrammatically in Figure 1 a pair ofcontacts made of my improved materials. In Figure 2 I have pictureddiagrammatically the processemployed for detecting the presence ofobjectionable materials and removing them from the contact metal.

The contacts illustrated at 2 are of the conventional type used on theinterrupters of 100 i ition coils used on automobile engines. li weverthe invention is applicable to all kinds of switches and make and breakdevices; in other words to all devices designed to effect interruptionof an electric circuit.

fication ofthe manufacturing processes involved in reducing the metal,refining it,

drawing and forming it, for even the slightest trace of any of the lowwork function 1 metals or any of their compounds will have anappreciable effect upon the sparking voltage. And in general, throughoutthe manufacturing process from the raw material to the finished roductcare must be taken that the undesirable low work function elements arenot introduced into the contact metal.

To insure that contact metal free of low work function elements isobtained, it will be found necessary to test the material preferably byemploying the same method of cooking under vacuum that may be used toremove the objectionable material when its presence has beenascertained. In many cases mere chemical analysis will not show thepresence of the impurities. The preferred method of testing isillustrated in Figure 2, where 3 is a. vacuum chamber, here shown in theform of a bell j ar.. Vacuum is maintained by connection at 4 to a pump,not shown, which may be the well known Gaede molecular type. At 6 I haveshown a. strip of the contact metal connected by suitable conductors tothe secondary side of a transformer 8, the primary being connected to asuitable source of power. 10 indicates a plate arranged in parallel withthe contact metal 6. In actual practice the contact metal and the plateare arranged as close together as possible to increase the electronflow. The plate is given a positive charge by connection to a storagebattery 12, the negative pole of which is connected to the secondaryside of the'transformer. 14 indicates an ammeter connected in serieswith the late 10 and battery.

Upon eating the contact metal 6 to a red heat electrons are given off inquantity and are attracted by the plate 10 because of its positivecharge. The amount of electron flow thus produced is measured by theammeter. By comparison of the ammeter reading with that obtained wherepure contact metal is used the amount of undesirable low work functionmetal may be determined. Should the reading indicate the presence of anundesirablylarge-trace of the metal, it is possible, by using the methoddescribed for 63 a suflicient time usually a few hours, to effect aremoval of the major portion of the material", since it leaves the wireat a greater rate than the base metal. When the ammeter readingindicates depletion of the low work function metal, the operation may bestopped, and the metal shaped into the desired form for contacts.

In some cases, little, if any, treatment may be necessary to remove theundesirable elements for the raw materials-used may be relatively pure,and the rocesses of manufacture may be so modi ed and conducted withsuch care as to prevent the introduction of the undesirable elementsinto the contact. However, in most cases, it will be found desirable totest" the metal as above indicated prior to employing it for contacts.In other cases, it may be necessary to cook the metal for several hoursto remove the undesirable material.

Owing to the higher sparking voltage obtained by the use of contactmetals made as described in this specification and the resultant shorterduration of the sparking, .it is apparent that my contacts will lastmuch longer than those made by ordinary manu facturing methods. Thiswill, in some cases, permit the substitution of cheaper metals for themore expensive spark resisting metals, such as, platinum, silver, andthe like, without decrease in serviceability. i

I claim:

1.-An electrical contact composed entirely of metal of high workfunction.

2. The method of preparing electrical contacts which consists inreducing the metal or alloy to a condition of relative purity,subjecting it to high vacuum adjacent a positively charged body toindicate the presence of material of low work function, removing saidmaterial, and fabricating the contact from the refined material.

3. The method of preparing electrical contacts which consists inreducing the metal or alloy to a condition of relative purity, testingthe metal for the presence of elements of low work function, removingthe said elements if their presence is indicated, and fabricating thecontacts from the metal thus refined.

4. The method of preparing high work function metal for use inelectrical contacts which consists in removing therefrom all traces oflow work function materials.

In testimony whereof I afiix my signature.

HECTOR ,RABEZZANA.

